1. Field of the Invention
The present invention relates to an engine. In particular, the invention relates to an engine having intake valves operated at a plurality of different intervals by a single camshaft.
2. Description of the Background Art
Types of the crankshaft of the V-type 8-cylinder engine include single-plane crankshaft where crankpins are arranged at an interval of 180° and double-plane (also referred to as dual-plane) crankshaft where crankpins are arranged at an interval of 90°. In the case where a V8 engine having a bank angle of 90° uses a crankshaft with crankpins of the single-plane arrangement, firing of the right bank and firing of the left bank can be caused alternately. In this case, since the four cylinders provided in each bank are fired at regular intervals (180° in crank angle), pulsation of exhaust gas is substantially constant. Therefore, adverse influences of the exhaust pulsation on an output (intake air quantity) are easily suppressed. Thus, the single-plane crankshaft is particularly employed for a vehicle for which output is of primary importance. In contrast, in the case where a crankshaft with crankpins of the double-plane arrangement is used, the good balance of the crankpin arrangement provides less vibrations of the crankshaft. Thus, the double-plane crankshaft is particularly employed for a vehicle for which riding comfort is of primary importance. However, in the case where the double-plane crankshaft is employed, it could occur that the right and left banks cannot be fired alternately and firing is successively caused in one of the banks. Therefore, in each bank, the firing cannot be caused at regular intervals and pulsation of exhaust gas is not constant. Accordingly, it is difficult to suppress adverse influences of the exhaust pulsation on the output (intake air quantity). For example, only a part of a plurality of cylinders is influenced by the exhaust pulsation, possibly resulting in a decrease in quantity of intake air (fresh air) into the cylinder. In this case, the output of the engine as a whole decreases. Accordingly, a technique has been proposed that suppresses a decrease in output due to different quantities of intake air into respective cylinders.
Japanese Patent Laying-Open No. 2003-056374 discloses a variable-valve control apparatus for an engine in which valve characteristics of intake and exhaust valves of each cylinder can be controlled independently of other cylinders, one exhaust pipe is connected to a group of cylinders and, in the group of cylinders, the cylinders are fired at different intervals. In the group of cylinders that are fired at different intervals, valve characteristics of at least one of the intake valve and the exhaust valve of a cylinder that is inferior in intake efficiency to other cylinders are changed so that the intake efficiency is improved. For at least one group of cylinders, the closing timing of the intake valve of a cylinder inferior in intake efficiency is corrected so that the intake air quantity increases. The intake valve and the exhaust valve are driven by an electromagnetic valve apparatus. By feeding electric power to an electromagnet used for closing the valve and feeding electric power to an electromagnet used for opening the valve to energize the magnets, or stopping the feeding, opening and closing of the intake valve and the exhaust valve are controlled.
Regarding the variable-valve control apparatus disclosed in the above-referenced publication, the closing timing of the intake valve that provides the maximum total intake gas quantity by intake inertia is a timing corresponding to a position retarded by a predetermined angle with respect to the bottom dead center. Then, the intake-valve closing timing is corrected so that the timing approaches the timing at which the total intake gas quantity is maximum, and thus the total intake gas quantity can be increased and the intake efficiency of the cylinder which is inferior in intake efficiency can be improved to be equivalent to that of other cylinders. In this way, the potential of the engine as a whole can be improved.
It is noted that the quantity of air sucked into a cylinder could also be influenced by any factor except for the exhaust pulsation. In a V8 engine in which cylinders in each bank are not fired at regular intervals, respective opening/closing timings (crank angles of opening/closing) of the intake valves provided to respective cylinders do not occur at regular intervals as well. Thus, there are the case where any intake valve in a bank is closing while another intake valve in the same bank is opening as well as the case where such intake valve timing does not occur. Here, if the intake valves are operated by the same camshaft, the timing (crank angle) at which the intake valve is closed is delayed for only a part of cylinders. This is because of an increased torque that is necessary for rotating the camshaft and thereby opening the intake valve and a resultant decrease in rotational speed of the camshaft. When the closing timing of the intake valve is delayed, the quantity of air pushed back from the cylinder into the intake manifold while the piston is rising increases, resulting in a decrease in quantity of air sucked into the cylinder. Therefore, the cylinders are nonuniform in terms of intake air quantity. However, since the variable-valve control apparatus for the engine disclosed in Japanese Patent Laying-Open No. 2003-056374 operates intake valves by means of electromagnetic force, no camshaft is provided. Thus, no consideration is given to decrease in rotational speed of the camshaft. Therefore, even if the variable-valve control apparatus for the engine disclosed in Japanese Patent Laying-Open No. 2003-056374 is used to correct the intake valve closing timing so that the timing approaches the timing at which the total intake gas quantity is maximum, the intake valve cannot be closed at the corrected timing. Therefore, a decrease in intake air quantity cannot be suppressed.